Restore old UI (Tampermonkey script)

// ==UserScript==
// @name         YouTubeUI Fix
// @namespace    http://tampermonkey.net/
// @version      20251022.14.45
// @description  Makes new UI look like an old one
// @author       You
// @license      No License
// @match        https://www.youtube.com/*
// @grant        GM_addStyle
// ==/UserScript==

(function() {
    'use strict';

    GM_addStyle(`
        :root {
            --toolbar-height: 52px;
            --element-margin: 6px;
            --element-height: calc(var(--toolbar-height) - var(--element-margin));
            --element-fs-margin: 2px;
            --bg-color: #0000;
        }

        /* SIZE */
        .ytp-delhi-modern .ytp-tooltip {
            margin-top: 12px !important;
        }

        .ytp-delhi-modern .ytp-chrome-bottom,
        .ytp-delhi-modern .ytp-chrome-controls,
        .ytp-delhi-modern.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-chrome-bottom,
        .ytp-delhi-modern.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-right-controls .ytp-right-controls-right,
        .ytp-delhi-modern.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-right-controls .ytp-right-controls-left {
            height: var(--toolbar-height) !important;
            line-height: var(--toolbar-height) !important;
        }

        .ytp-delhi-modern.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-progress-bar-container,
        .ytp-delhi-modern:not(.ytp-player-minimized) .ytp-progress-bar-container {
            bottom: var(--toolbar-height) !important;
        }

        /* Overlays container */
        .ytp-delhi-modern.ytp-big-mode.ytp-fullscreen-grid-peeking .ytp-overlays-container,
        .ytp-delhi-modern.ytp-fullscreen-grid-peeking .ytp-overlays-container {
            bottom: calc(var(--toolbar-height) + 20px) !important;
        }

        .ytp-delhi-modern .ytp-chrome-controls .ytp-right-controls,
        .ytp-delhi-modern .ytp-prev-button:not(.ytp-miniplayer-button-container > *),
        .ytp-delhi-modern .ytp-chrome-controls .ytp-next-button:not(.ytp-miniplayer-button-container > *).ytp-playlist-ui {
            margin: var(--element-margin) 0 !important;
            padding: 0 !important;
        }

        .ytp-delhi-modern .ytp-chrome-controls .ytp-play-button {
            margin: var(--element-margin) 0 !important;
        }

        /* Volume */
        .ytp-delhi-modern.ytp-delhi-horizontal-volume-controls .ytp-volume-area {
            margin: var(--element-margin) 0 !important;
        }

        .ytp-delhi-modern .ytp-time-display:not(.ytp-miniplayer-ui *),
        .ytp-delhi-modern .ytp-chapter-container {
            padding: var(--element-margin) 0 !important;
        }

        /* Play button */
        .ytp-delhi-modern .ytp-chrome-controls .ytp-play-button {
            --size: var(--element-height);
            height: var(--size) !important;
            width: var(--size) !important;
            top: -2px;
        }

        /* Play icon */
        .ytp-delhi-modern-icons .ytp-chrome-controls .ytp-play-button svg {
            padding: 10px !important;
            --size: 26px;
            height: var(--size) !important;
            width: var(--size) !important;
        }

        /* gradient */
        .ytp-chrome-bottom:after {
            content: '';
            position: absolute;
            left: -100px;
            bottom: -2px;
            width: calc(100% + 200px);
            background: linear-gradient(0deg, #000a, #0000);
            height: calc(var(--toolbar-height) + 10px);
            z-index: -1;
        }

        /* fullscreen fixes */
        .ytp-delhi-modern.ytp-delhi-horizontal-volume-controls.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-volume-area,
        .ytp-big-mode .ytp-volume-slider,
        .ytp-delhi-modern .ytp-chrome-controls .ytp-mute-button,
        .ytp-delhi-modern.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-time-display,
        .ytp-delhi-modern.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-chrome-controls .ytp-next-button:not(.ytp-miniplayer-button-container > *),
        .ytp-big-mode.ytp-delhi-modern .ytp-chrome-controls .ytp-prev-button:not(ytp-miniplayer-button-container > *),
        .ytp-delhi-modern.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-time-wrapper,
        .ytp-delhi-modern.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-chapter-title.ytp-button {
            height: var(--element-height) !important;
            min-height: var(--element-height) !important;
            line-height: var(--element-height) !important;
            margin-top: 0px !important;
            margin-bottom: 0px !important;
            --element-margin: 2px;
        }

        .ytp-big-mode.ytp-delhi-modern-icons .ytp-chrome-controls .ytp-button svg,
        .ytp-delhi-modern.ytp-big-mode:not(.ytp-xsmall-width-mode) .ytp-chrome-controls .ytp-next-button:not(.ytp-miniplayer-button-container > *) svg,
        .ytp-big-mode.ytp-delhi-modern .ytp-chrome-controls .ytp-prev-button:not(ytp-miniplayer-button-container > *) svg {
            padding-top: 14px;
        }

        .ytp-big-mode.ytp-delhi-modern.ytp-delhi-horizontal-volume-controls .ytp-volume-area .ytp-volume-icon svg {
            padding-top: 10px;
        }

        /* COLORS */
        .ytp-delhi-modern .ytp-time-wrapper:not(.ytp-miniplayer-ui *),
        .ytp-delhi-modern.ytp-delhi-horizontal-volume-controls .ytp-volume-area,
        .ytp-delhi-modern .ytp-chrome-controls .ytp-play-button,
        .ytp-delhi-modern .ytp-chrome-controls .ytp-right-controls,
        .ytp-delhi-modern .ytp-chrome-controls > * > *,
        .ytp-delhi-modern .ytp-chapter-title.ytp-button {
            background: var(--bg-color) !important;
        }

        /* Play and fullscreen buttons clickable in corners */
        .ytp-fullscreen-button {
            overflow: unset !important;
        }

        .ytp-fullscreen-button::after {
            content: '';
            position: absolute;
            inset-block: -10px;
            inset-inline: 0px;
            width: 220px !important;
            height: 200px;
        }

        /* Video endscreen fix */
        .ytp-fullscreen-grid-active.html5-video-player.ended-mode .ytp-modern-videowall-still:nth-child(n+4) {
            display: block !important; /* Show thumbnails starting from the 4th one */
        }

        .ytp-fullscreen-grid-active.html5-video-player.ended-mode .ytp-modern-videowall-still:nth-child(n+11),
        .ytp-fullscreen-grid-expand-button {
            display: none !important; /* Hide thumbnails after the 10th one and the expand button */
        }

        .ytp-fullscreen-grid-active.ytp-grid-scrollable .ytp-fullscreen-grid {
            top: 10% !important; /* Adjust grid positioning */
        }

        .ytp-fullscreen-grid-stills-container {
            grid-template-columns: repeat(5, 1fr) !important; /* Use 5 columns for grid */
        }

        ytd-app {
            overflow: auto !important;
        }

        ytd-app[scrolling] {
            position: absolute !important;
            top: 0 !important;
            left: 0 !important;
            right: calc((var(--ytd-app-fullerscreen-scrollbar-width) + 1px) * -1) !important;
            bottom: 0 !important;
            overflow-x: auto !important;
        }

        ytd-watch-flexy[full-bleed-player] #single-column-container.ytd-watch-flexy,
        ytd-watch-flexy[full-bleed-player] #columns.ytd-watch-flexy {
            display: flex !important;
        }

        .ytp-fullscreen-grid-peeking.ytp-full-bleed-player.ytp-delhi-modern:not(.ytp-autohide) .ytp-chrome-bottom {
            bottom: 0 !important;
            opacity: 1 !important;
        }

        #movie_player:not(.ytp-grid-ended-state) .ytp-fullscreen-grid {
            display: none !important;
            top: 100% !important;
            opacity: 0 !important;
        }
    `);
})();